1. Field of the Invention
The present invention relates to an electroacoustic transducer, such as an electromagnetic type electroacoustic transducer or a piezo type electroacoustic transducer, and, more particularly, to an electroacoustic transducer which permits control of the spring characteristic of a diaphragm used therein to stabilize the resonance frequency (f.sub.0), thereby improving the follow-up reproducibility of an output to an input.
2. Description of the Related Art
An electromagnetic type electroacoustic transducer may have a structure as shown in FIG. 30. This electroacoustic transducer has a case 1 and a base 3 located within the case 1 at the bottom portion in the diagram. A core 5 is mounted in the center of the base 3. A bobbin 7 is mounted outside the core 5, with a magnet wire 9 wound around the bobbin 7. Attached to the bobbin 7 are terminals 11 and 13 extending downwardly in the diagram. A plastic magnet 15 is disposed between the outer surface of the bobbin 7 and the inner wall of the case 1. The bottom surface of the base 3 is coated with a potting agent 17 to enhance the sealing performance.
An elastic plate 19 is disposed facing the core 5 in the case 1. This elastic plate 19 is adsorbed to the plastic magnet 15. A magnetic piece 21 as an added mass is attached to the center portion of the elastic plate 19. The elastic plate 19 and magnetic piece 21 constitute a diaphragm 20. The main purpose of attaching the magnetic piece 21 as an added mass is to set the frequency of an output sound lower by increasing the mass. A through hole 23 is provided in the center of the top of the case 1 in the diagram. A masking label 25 is provided to cover the through hole 23. The elastic plate 19 and magnetic piece 21 are constituted as shown in FIGS. 31 and 32. As apparent from those diagrams, the elastic plate 19 and magnetic piece 21 are shaped like a disk, the magnetic piece 21 secured at its center to the elastic plate 19 by spot welding. The securely welded portion is denoted by a numeral "27" in FIG. 31. The line specified by a numeral "29" in FIG. 31 is a mark to discriminate one side of the elastic plate 19 from the other. That is, one side of the elastic plate 19 has its edge portion rounded to have a roll-over face and the other side has its edge portion roughened to have a burr face. The magnetic piece 21 is attached to the roll-over face side of the elastic plate 19. The line or mark 29 is provided to discriminate the roll-over face from the burr face.
With the above structure, the elastic plate 19 is attracted and adsorbed to the plastic magnet 15, so that it is set to have a given polarity. When a current flows across the coil via the terminals 11 and 13 under this condition, the core 5 is electrically magnetized, generating a magnetic field at the distal end. At this time, when the magnetic field generated on the core 5 by the excited coil has a different polarity than the magnet-oriented polarity of the elastic plate 19, the elastic plate 19 is attracted to the core 5. When the polarity of the magnetic field of the core 5 is the same as that of the elastic plate 19, on the other hand, the elastic plate 19 moves away from the core 5. Intermittent current supply to the coil in a given direction causes the elastic plate 19 to repeat the up and down movement to the core 5, thus vibrating at a given frequency. This vibration generates a sound.
According to the conventional structure, however, the resonance frequency (f.sub.0) does not become stable in some cases, thus lowering the output-to-input follow-up reproducibility. In other words, the compliance of the diaphragm 20 (the reciprocal of the spring constant or the spring characteristic of the diaphragm) is determined by the compliance of that portion of the elastic plate 19 which is not in contact with the magnetic piece 21, the state of the contact portion between the elastic plate 19 and the outer periphery of the magnetic piece 21, and the state of that portion which supports the elastic plate 19 (the state of the attracting/adsorbing structure by the plastic magnet 15 in the conventional structure). Further, tension is produced by the plastic magnet 15, on the elastic plate 19 around the outer periphery of the magnetic piece 21 and around the outer periphery of the elastic plate 19 which is supported by the attraction or adsorption of the plastic magnet 15. This tension makes the compliance smaller.
If the force of securing the outer surface portion of the diaphragm 20 by the attraction/adsorption of the plastic magnet 15 is strong enough, when the diaphragm 20 vibrates, the tension increases with an increase in the amplitude of the diaphragm 20 (an increase in the applied voltage) due to the aforementioned action. This reduces the compliance. That is, the spring constant of the diaphragm 20 gradually becomes greater. As the amplitude increases, therefore, the resonance frequency (f.sub.0) rises as shown in FIG. 33, so that the spring characteristic of the diaphragm 20 becomes that of a hard spring system. If the force of securing the outer surface portion of the diaphragm 20 by the attraction/adsorption of the plastic magnet 15 is weak, on the other hand, the following will occur. When the amplitude of the diaphragm 20 increases, the outer periphery of the elastic plate 19 starts moving away from the plastic magnet 15 with the inner contact portion of the elastic plate 19 to the plastic magnet 15 being a fulcrum due to the small attraction/adsorption force. Accordingly, the tension decreases, resulting in temporary increase in the compliance. When the amplitude further increases later, the tension increases, causing the increased compliance to fall. This situation is illustrated in FIG. 34. In short, the resonance frequency (f.sub.0) rises after temporary fall. In either case, there is a region where the resonance frequency (f.sub.0) varies with a change in the amplitude of the diaphragm 20 (the shaded portions in FIGS. 33 and 34), and the output-to-input follow-up reproducibility is deteriorated in that region. When an electromagnetic type electroacoustic transducer with the above structure is used for an amplitude-modulated (AM sound) or an attenuating sound, there occurs a region where the desired sound pressure and/or timbre to an input cannot be reproduced.
This shortcoming occurs not only in the electromagnetic type electroacoustic transducer, but also in a piezo type electroacoustic transducer. In the piezo type electroacoustic transducer, an added mass is also attached to a piezo element or elastic plate in some cases to set the frequency of an output sound lower by the increased mass. In this case, the compliance of the portion around the added mass and that of the portion around the to-be-supported portion decrease with an increase in the amplitude of the diaphragm, causing the same problem as occurred in the case of the electromagnetic type electroacoustic transducer.
The prior art solutions to the above problem are disclosed in Examined Japanese Utility Model Publication No. 51-43807 (Reference 1), Examined Japanese Utility Model Publication No. 51-43808 (Reference 2), and Unexamined Japanese Patent Publication No. 60-220397 (Reference 3). References 1 and 2 disclose an art of using a thin layer as an added mass to provide elasticity to the outer peripheral portion of the added mass, so that a change in the compliance is suppressed when the amplitude of the diaphragm changes. In this case, however, dew condensation, freezing or rust will cause a large change in the compliance of the vibrating system. Reference 3 discloses an art of making that face of an added mass which contacts an elastic plate to have a curved surface with a curvature matching the curvature of the elastic plate in the attracted condition, thereby suppressing a rise of the resonance frequency (f.sub.0) in the attracted condition, and suppressing a change in the compliance with an change in the amplitude of the diaphragm. But, in this prior art device, it is difficult to design the curved surface of the added mass and control the mass production of electroacoustic transducers.